Poles and Germans even got along after the partitions.

Of course the Germanization of Poles happened, especially after 1873.

The process of Polonization (Lithuania, Ukraine, etc.) has a rich history, because Poland was part of a multi-ethnic state:

https://en.wikipedia.org/wiki/Polonization

The Lithuanian National Revival, for example, was a consequence of Polonization, with a lot of conflicts between Lithuanians and Poles still existing today.

Prussians actually respected the Polish culture and turned their German culture into Polish to accomodate the rulers:

"Since 1527 there have been complaints from representatives of large cities that some council members use Polish, although they know German. In 1555, a canon of Gniezno delivered a speech to the Prussian Sejm in Polish, without the help of an interpreter. In the second half of the 16th century, royal decrees were issued in Polish, debates in the Landtag were held in Polish. Great Prussian families polonised their names: the Baysen to Bażyński; the Zehmen to Cema; the Dameraw to Działyński, and the Mortangen to Mortęski, the Kleinfelds to Krupocki."

The fact that a lot of the Polonization processes actually happened after the partition, shows that this process was somewhat independent of the concept of cultural unity on a political level.

Even after the partition, many Germans turned Polish:

"It is estimated by the German that during the nineteenth century 100,000 Germans in the eastern provinces of Prussia were Polonized, that is, they adopted the language, religion, and sentiments of the Poles. During this time the Poles were making no systematic effort in this direction. It seems to me that the main force in operation was the attractive qualities of the Poles -- and their more intimate, personal, face-to-face relations."

While many Poles voluntarily subordinated themselves to German:

"On the other hand it seems that the Polish population was at one time on the road to Germanization. In the period of serfdom the peasant had been so mercilessly exploited that he acquired a profound suspicion of the upper classes, and this remains a prominent trait in his character today. (...) And under the German government he began to be loyal (for Germany understands how to care for her people) and for a long time -- until after the war with France -- she treated the Poles without discrimination -- protected them and let them alone. And they in turn began to be patriotic, to speak German and drink beer, and to be proud of the Prussian uniform."

https://web.archive.org/web/20020730233334/http://spartan.ac...

Germanization of Poles happened, just like Polonization of Germans happened in the Polish areas. But until the beginning of nationalism, this was mostly a voluntary processs neither forced by rulers nor directed at civilians.

Only at the end of the 19th century, with forced Germanization policies by Bismarck and long after the Congress of Vienna, did the resentments start:

"But in 1873 he (the peasant) was attacked by the government. At this point Bismarck took a hand and decided to force the process of Germanization. (...) There is not the slightest doubt that the Prussian government at this point raised a devil which it has not been able to lay. This action, indeed, marked the beginning of what is now known as the Polish Peasant Republic in Posen. The direct consequences of this school policy were riots and school strikes. "